Conventional gas turbine engines with can annular combustors have transition ducts that receive hot gases from the combustor and direct them to a first row of turbine vanes. Upon entering the first row of turbine vanes the hot gases are accelerated from approximately 0.2 mach to approximately 0.8 mach, which is an appropriate speed for delivery onto a first row of turbine blades. The transition duct is disposed inside a plenum that receives compressed air from a compressor and delivers it to an inlet of the combustor. The transition duct separates the compressed air in the plenum from the combustion gases in the transition duct. The compressed air in the plenum is moving more slowly than the hot gases and as a result there is a static pressure difference across the transition duct that produces mechanical forces that the transition duct must withstand. Conventional transition ducts of simple tubular design have been able to withstand these relatively mild mechanical forces while remaining thin enough to permit necessary cooling.
The necessary cooling may be effected in many ways, one of which includes placing a flow sleeve around the transition duct. This creates a flow path between the two through which a cooling fluid may flow. This cools the outer surface of the transition ducts enough to ensure long service life. Film cooling holes may be disposed through the transition duct which will permit a film of cooling air to develop between an inner surface of the transition duct and the hot gases, which will also improve the service life of the transition duct.
Certain emerging technology gas turbine engine combustor system designs have a new ducting arrangement that receives a flow of hot gases from each combustor and delivers each flow along a straight flow path directly onto the first row of turbine blades. Various embodiments may unite the discrete hot gas flows in a common chamber immediately upstream of the first row of turbine blades. In these new ducting arrangements the traditional first row of turbine vanes is dispensed with. The role of accelerating the hot gases from 0.2 mach to 0.8 mach has been transferred from the traditional first row of turbine vanes to the ducting structure itself. One example of such an emerging technology combustor is disclosed in U.S. Patent Application Publication Number 2011/0203282 to Charron et al. and is incorporated herein by reference.
The new ducting structure must withstand significantly greater mechanical forces induced by the static pressure difference. The compressed air in the plenum is traveling at approximately the same speed as in the conventional gas turbine engines, but the hot gases traveling through the ducting arrangement are traveling at speeds approaching approximately 0.8 mach, which is nearly 4 times faster than the speed of the hot gases within the traditional transition ducts. The static pressure difference created by the greater difference in speed of the compressed air in the plenum (outside the ducting arrangement) and the hot gases in the ducting arrangement is therefore much greater. As a result, the new ducting arrangement must withstand much greater mechanical forces induced by the greater difference in static pressure.
Stronger designs that are still thin enough to permit sufficient cooling are being considered to enable the ducting arrangement to withstand the greater mechanical forces. Compatible cooling arrangements are needed to accommodate the stronger designs, and thus there is room for improvement in the art.